The present application is related to audio signal processing and particularly to audio processing of a mono or dual-mono signal.
An auditory scene can be modeled as a mixture of direct and ambient sounds. Direct (or directional) sounds are emitted by sound sources, e.g. a musical instrument, a vocalist or a loudspeaker and arrive on the shortest possible path at the receiver, e.g. the listener's ear or a microphone. When capturing a direct sound using a set of spaced microphones, the received signals are coherent. In contrast, ambient (or diffuse) sounds are emitted by many spaced sound sources or sound reflecting boundaries that contribute to, for example, room reverberation, applause or a babble noise. When capturing an ambient sound field using a set of spaced microphones, the received signals are at least partially incoherent.
Monophonic sound reproduction can be considered appropriate in some reproduction scenarios (e.g. dance clubs) or for some types of signals (e.g. speech recordings), but the majority of musical recordings, movie sound and TV sound are stereophonic signals. Stereophonic signals can create the sensation of ambient (or diffuse) sounds and of the directions and widths of sound sources. This is achieved by means of stereophonic information that is encoded by spatial cues. The most important spatial cues are inter-channel level differences (ICLD), inter-channel time differences (ICTD) and inter-channel coherence (ICC). Consequently, stereophonic signals and the corresponding sound reproduction systems have more than one channel. ICLD and ICTD contribute to the sensation of a direction. ICC evokes the sensation of width of a sound and, in the case of ambient sounds, that a sound is perceived as coming from all directions.
Although multichannel sound reproduction in various formats exist, the majority of audio recordings and sound reproduction systems still have two channels. Two-channel stereophonic sound is the standard for entertainment systems, and the listeners are used to it. However, stereophonic signals are not restricted to have only two channel signals but can have more than one channel signal. Similarly, monophonic signals are not restricted to have only one channel signal, but can have multiple but identical channel signals. For example, an audio signal comprising two identical channel signals may be called a dual-mono signal.
There are various reasons why monophonic signals instead of stereophonic signals are available to the listener. First, old recordings are monophonic because stereophonic techniques were not used at that time. Secondly, restrictions of the bandwidth of a transmission or storage medium can lead to a loss of stereophonic information. A prominent example is radio broadcasting using frequency modulation (FM). Here, interfering sources, multipath distortions or other impairments of the transmission can lead to noisy stereophonic information, which is for the transmission of two-channel signals typically encoded as the difference signal between both channels. It is common practice to partially or completely discard the stereophonic information when the reception conditions are poor.
The loss of stereophonic information may lead to a reduction of sound quality. In general, an audio signal comprising a higher number of channels may comprise a higher sound quality when compared to an audio signal comprising a lower number of channels. Listeners may listen to audio signals comprising a high sound quality. For efficiency reasons such as data rates transmitted over or stored in media sound quality is often reduced.
Therefore, there exists a need for increasing (enhancing) sound quality of audio signals.